As a method for coating metallic materials as iron plate, galvanized steel sheet, aluminum plate and the like, the socalled 2 coat-2 bake system has generally been used, in which the metallic material, after being subjected to a chemical treatment, is first coated with primer and then with a top-coat. The thus prepared precoat metals have been widely used in various areas, including construction materials, and therefore, are required to have higher order of weathering resistance, corrosion resistance, processability, water resistance or the like. Especially, the abovesaid primer coating should have excellent adherence to both substrate and top-coat, as well as improved processability, corrosion resistance, water resistance, chemical resistance and the like.
Heretofore, as a primer paint, use has generally been made of a heat curing type composition comprising as principal components an epoxy resin of the formula: ##STR1## (wherein n is 0 or an integer of 1 to 14) and amino resin and/or blocked isocyanate compound. Such composition can provide a tough coating with improved corrosion resistance and chemical resistance, and however, has a drawback of poor processability. For the purpose of increasing the processability, attempts have been made to use a high molecular weight epoxy resin or to use polyhydroxy-polyether-polyester obtained by the reaction of relatively lower molecular weight epoxy resin with dicarboxylic acid. However, in the former, indeed corrosion resistance and processability are improved, but additional problems are caused to occur as poor solubility in solvents and poor compatibility with other components of the coating composition. In the latter case, there are certain limitations in the selection of the hardners, because of its relatively poor compatibility with blocked isocyanate compound, and hence cannot exhibit film performance in full in such combination. Furthermore, in the reaction of epoxy resin with dicarboxylic acid, there often occur, besides the addition reaction of glycidyl groups and carboxyl groups, side reactions, i.e. dehydrating condensation of said carboxyl groups and hydroxyl groups originally presented in the epoxy resin or newly generated by the reaction of said glycidyl groups and carboxyl groups thereby resulting the product with many branched chains.
The resinous varnish thus obtained can hardly give the desired processability and in extreme case, the varnish itself, will be gelated thoroughly.
By the adoption of decreased reaction temperature, such phenomena may be avoided, but in that case a prolonged reaction time is required, which is untolerable in commercial production of the intended product. It is, of cause, possible to use a catalyst as tertiary amine, potassium hydroxide, sodium hydroxide, quaternary ammonium salt and the like to shorten the reaction time, and however, in this time, the catalyst used may remain in the produced resin and cause the problems in water resistance and chemical resistance of the coating.
When the following type epoxy resin: ##STR2## is used in place of the abovesaid resin (A), the hydroxyl groups contained in the resin and the groups newly generated by the reaction with dicarboxylic acid are all of tertiary nature and hence, there hardly occur the dehydrating condensation of carboxyl groups and hydroxyl groups.
Therefore, the desired polyhydroxy-polyether-polyester may be comparatively stably prepared. However the tertiary nature of the hydroxyl groups in the final resin will also give a difficult question of inferior reactivity with the functional groups of amino resin and/or blocked isocyanate compound, and therefore, a higher temperature baking is essential for the thorough crosslinking and, which is undesired from an economical and operational point of views.
Heretofore, has also been reported a heat curing type epoxy resin for primer use (Japanese Patent Application Kokai No. 30717/82).
This resin is prepared by the interaction of
(1) the reaction product of epi-bis type epoxy resin (A) and/or (B), dicarboxylic acid and secondary amine with primary hydroxyl group, and PA0 (2) the reaction product of polyisocyanate compound and isocyanate blocking agent. PA0 (a.sub.2) dicarboxylic acid and PA0 (a.sub.3) secondary amine having at least one primary hydroxyalkyl group, PA0 and product [B] which is obtained by the reaction of PA0 (b.sub.1) polyisocyanate compound and PA0 (b.sub.2) isocyanate blocking agent, PA0 (a.sub.1) an epoxy resin component composed of 0.about.95% by weight of epoxy resin of the formula: ##STR14## wherein R is H or CH.sub.3, --A-- represents &gt;C(CH.sub.3).sub.2, --CH.sub.2 --, --O--, --S-- or --SO.sub.2 --, and n is 0 PA0 (a.sub.3) a secondary amine having at least one primary hydroxyalkyl group, and product [B]' obtained by the reaction of PA0 (b.sub.1) a polyisocyanate compound and PA0 (b.sub.2) an isocyanate blocking agent,
The abovesaid secondary amine may act as a catalyst for the addition reaction of glycidyl group of the epoxy resin and carboxyl group of the dicarboxylic acid, thereby enabling the reaction to proceed at a lower temperature and at a higher speed, and also have the role of introducing primary hydroxyl groups in the resin, thereby facilitating the progression of crosslinking reaction when compounded with hardener.
Therefore, the coating composition based on such resin, may give combination of excellent as adherence, processability, corrosion resistance, water resistance, and chemical resistance, and is quite useful as primer coating for metallic materials. However, with the diversification of needs, the appearance of primer paint capable of showing far improved adherence, bending property and especially corrosion resistance at processed portions has been longed for.
In a series of studies of having improved epoxy type primer coat, especially with respect to corrosion resistance, the inventors have found that in the heat curing type epoxy resin obtained by the interaction of the reaction product of epoxy resin, dicarboxylic acid and primary hydroxy bearing secondary amine, and the reaction product of polyisocyanate compound and isocyanate blocking agent, the adherence, bending property and corrosion resistance of the coating are greatly improved by the use of epoxy resin with a resorcin structure as a part of said epoxy resin or by combining the abovesaid heat curing type epoxy resin with polyether resin with a resorcin structure, and in a conventional type epoxy resin, the corrosion resistance of the coating can be greatly improved by the inclusion of particular acid in said epoxy resin in state capable of developing particular resinous acid value or by converting the epoxy resin to an amphoteric type through the reaction with polybasic acid and particular alkyleneimine compound. On the basis of these findings, the invention has been made.